The present invention relates to turbochargers having a variable-geometry mechanism for regulating the flow area of the nozzle leading into the turbine, and having a waste gate for causing exhaust gas to bypass the turbine under certain conditions.
In turbochargers of the above-described type, the variable-geometry mechanism (which can comprise a sliding piston or slide vane, or variable vanes in the nozzle, or the like) is typically movable between a fully closed position and a fully open position. In the fully closed position, the nozzle is significantly closed (and in some cases completely closed) by the mechanism. In the fully open position of the piston, the nozzle is significantly open (and in some cases completely open).
The variable-geometry mechanism can be difficult to control at the very beginning of the opening process. When the mechanism is opened just slightly from its fully closed position, it can create a significant amount of disturbance in the exhaust gas flow into the turbine, thereby decreasing efficiency of the turbocharger.